<h2>DESCRIPTION</h2>

<em>r.contour</em> produces a vector map of specified contours from input raster map.

Contours can be produced using a comma-separated list of values
in <b>levels</b>, or at some regular increment using the <b>step</b>
parameter, using <b>minlevel</b> and <b>maxlevel</b> as minimum and
maximum contour values, respectively. If no <b>minlevel</b>
or <b>maxlevel</b> is specified, the minimum and maximum cell values
in the <b>input</b> raster map will be used.

<h2>NOTES</h2>

<em>r.contour</em> will either step through incremental contours or produce
contours from a list of levels, not both. If both a list of levels and
a step are specified, the list will be produced and the step will be ignored.

<p>Zero is treated as a valid data value by <em>r.contour</em>.

<p>If a contour level exactly matches a category value in the raster map,
the contour line may backtrack on itself, causing illegal arcs to be produced
in the output vector map.

<p>The optional <b>cut</b> parameter allows the user to specify a minimum number of
raster cells eligilble to be included in a contour line written to the <b>output</b> 
vector map. It acts like a filter, omitting spurs, single points, etc., making the output more generalized.

<h2>EXAMPLES</h2>

In the Spearfish location, produce a vector contour map from input raster <i>elevation.dem</i> 
with contour levels from 1000m to 2000m, 100m contour step, and a minimum of 200 input raster
points contributing to the contour line:

<div class="code"><pre>
r.contour input=elevation.dem output=elevation_dem_contours \
          minlevel=1000 maxlevel=2000 step=100 cut=200
</pre></div>

<p>
For an example of using levels with <em>r.contour</em> this example uses
the elevation map from the North Carolina database:

<div class="code"><pre>
g.region raster=elevation
r.contour in=elevation out=contours levels=60,90,120,150 --o
d.mon wx0
d.rast elevation_shade
d.vect contours color=red
</pre></div>

<div align="center" style="margin: 10px">
<a href="r_contour_using_levels.png">
<img src="r_contour_using_levels.png" width="600" alt="r.contours example" border=0><br>
</a>
<i>Contour lines shown on shaded terrain map</i>
</div>


<p>
For an example of using steps with <em>r.contour</em> this example uses
the LiDAR data derived elevation map from the North Carolina database:

<div class="code"><pre>
g.region raster=elev_lid792_1m -p
r.contour input=elev_lid792_1m output=elev_lid792_1m_contours \
          minlevel=100 maxlevel=150 step=10
</pre></div>

<h2>AUTHORS</h2>

Terry Baker, U.S. Army Construction Engineering Research Laboratory<br>
3/2001: cut parameter and fixes by Andrea Aime (aaime@libero.it)

<!--
<p>
<i>Last changed: $Date$</i>
-->

